This study is intended to determine at the molecular level the precise basis for the antigenicity of cytochrome c as a model system for globular proteins. Such a large set of homologous proteins is uniquely suited to this objective because, on the one hand, the amino acid sequences of over 80 eukaryotic cytochromes c are known, and on the other hand, their 3-dimensional structures are essentially identical, perturbed only by the presence of different residue side chains. It has been possible to isolate single determinant antibody populations and to study in detail the relationship between the immunogenic structure and the response it evokes. The corresponding determinants are topographic, the specificity of the antibody populations being determined by the nature of the local amino acid side chains. Remote topographic determinants in the same molecule are independent one of another and the same sites on cytochromes c differing at other sites are indistinguishable. Cytochromes c with few amino acid sequence differences have so far been employed and the work will be extended to more complex responses. The concepts developed in this work should enable the detailed prediction of the antigenic structure of other proteins homologous to those of the host. This capability will be tested by analyzing the antibody responses to the myoglobins of various species. Antigenic sites identified in the above studies are determinants recognized by B-cells, and a similar approach will be employed to identify those determinants recognized by T-cells, using an assay measuring the proliferation of primed T-cells in vitro in response to various cytochromes c. If it is shown that both types of cell recognize the same determinants on cytochrome c, the nature of their respective surface receptors will be investigated with anti-idiotypic antisera elicited by site-specific antibodies. These studies should make it feasible to examine the interaction of immune lymphocyte populations specific to various single determinant sites on the antigen, providing information on some of the mechanisms of control of the immune response.